A teacher is almost everywhere when we were still young. They know things that we don't, heck they even tell us things that we don't understand. Now that we're bigger we realise that all they thought us was easy, and we just didn't really listen. I am now a teacher, and I keep telling them things that I feel they can understand but they just don't.
Conclusion is being a teacher is hard enough work as it is. Kudos to every single teacher you know. Heck. Go back to your grade school teachers and thank them for their patience. They loved you, you know!